Electron impact single ionization cross sections of copper have been calculated in the binary encounter approximation using accurate expression for σ_ΔE as given by Vriens and Hartree-Fock momentum distribution for the target electron. The BEA calculation based on the usual procedure does not show satisfactory agreement with experiment in this case but a striking modification is found to be successful in explaining the experimental observations. The discrepancy is linked with the ionization of the 3d¹⁰ electrons and probably effective single ionization does not take place from 3d shell of copper leading to smaller values of experimental cross sections.